The invention relates to a pistol for the pressing out of bone cement, comprising an attachable cement syringe which has a cylinder with an ejection piston and a narrower neck which adjoins at the cylinder, with a displacer bar being insertable into the pistol, which displacer bar can after the ejection of the cylinder content be introduced into the neck through an aperture of the ejection piston by means of a first tubular bar which acts on the ejection piston and which has second advance mechanism which is journalled in the tubular bar.
Cement pistols for the pressing out of bone cement from a cement syringe have been in use for several years. As a rule the cement syringe is attached to the pistol and then a bar is advanced via a trigger lever against the ejection piston of the cement syringe in order to eject the cement. A ratchet pawl design of this kind is shown in a brochure of the company DePuy, International Ltd., St. Antony""s Road, Beeston, Leeds, GB LS11 8DT under the title CMW MKIII ZEMENTPISTOLE (1363-024). It has the special feature that the bar which is actuated via the trigger lever and the ratchet pawl consists of a tube which is provided with a toothing, at the end of which a window is provided. When this window reaches the trigger mechanism the latter grips through the window into the toothing of a further bar which is journalled in the tube and drives this bar further forwards relative to the tube. In cement syringes which have a neck which adjoins at the cylinder the second bar can thus eject the volume in the neck with a displacer bar which is inserted into the tube when the piston which is actuated by the tube has arrived at the end of its path. The apparatus has the disadvantage that it is unwieldy because the two bars project rearwardly by their full length at the beginning of the ejection. A further disadvantage is that the operator must perform the pressing out work by hand. Even though the movement of the bars is stepped down by the trigger lever he must nevertheless actuate the latter many times.
In the patent specification U.S. Pat. No. 5,514,135 a throw-away bone cement syringe with an integrated pistol grip and a CO2 cartridge which is provided in the grip is shown. A small pressure chamber is displaced with the trigger lever from the CO2 side to the charging side of a free piston in order to supply compressed CO2 portion-wise to the piston. The bone cement is filled in through a closeable opening which is provided in the jacket surface of the cement syringe and can be ejected after the closing of this opening via a further outlet opening. A disadvantage of this arrangement consists in that it must be sterilizable with respect to the choice of materials, but can be used only once. A further disadvantage consists in that there exists here the risk of a non recognizable gas breakthrough to the bone cement when the piston binds and/or its sealing ring fails with respect to gas tightness. A gas breakthrough is not tolerable even in small amounts.
The object of the invention is to create an easy to handle cement pistol which is suitable for different ejection conditions. This object is satisfied in accordance with patent claim 1 in that the tubular bar is designed as a piston rod of a first piston which is charged in the pistol by a fluid; and in that the second advance mechanism has a second piston which is charged by the fluid and which ejects the displacer bar, with the two pistons being excitable with the fluid, which is under pressure, via a control device in the pistol.
Advantageous further developments of the invention result through the subordinate claims 2 to 14.
In order to determine the feasibility of a versatilely usable pistol for the pressing out of bone cement, in particular for compressible gaseous fluids, experiments were required.
For a cement syringe with a long attached neck, as is used in the xe2x80x9cretrogradexe2x80x9d filling up of a thigh bone marrow chamber, for example the following data resulted:
For a cement syringe with a short neck, such as is used in the xe2x80x9cantegradexe2x80x9d filling up of a bone cavity, for example the following data resulted:
For the pressing out of the bone cement out of a long attached neck with a displacer bar there resulted for example a force of 25 Newtons.
The control device and both the first and the second piston, which are charged by fluid, are arranged and dimensioned such that the above ejection conditions can be fulfilled. The fluid can be a liquid or a gas. These can be conducted to the pistol grip and away again externally via hoses. The handling becomes simpler when the reservoir with the fluid is integrated into the pistol for example in the form of gas cartridges in the grip part. CO2 cartridges have the advantage that the fluid can be introduced as a liquid with the cartridge and takes up little volume. In addition the pressure in the reservoir does not fall off too rapidly as long as liquid CO2 is still present.
The invention has the advantage that the operator can concentrate fully on the actual cement application. In addition the device can be used for different cement syringes, i.e. with necks of different lengths and with or without a displacer bar.
Furthermore, it is necessary that the ejection movement of the second piston be initiated only when the first piston has brought the ejection piston into its end position. In a triggering of the second piston which is too early, the latter would close off the outflow in the neck of the cement syringe via the displacer bar. A solution provides for using the displacer bar itself as a restoring mechanism. When the stroke of the first piston is greater than the possible displacement path of the ejection piston and an end-side gap between the piston rod and the ejection piston is present, the full ejection force between the displacer bar and the ejection piston can be produced over the width of this gap. This ejection force suffices in order to push a membrane or flap which is arranged in the passage opening for the displacer bar to one side with the displacer bar and to push the latter with the second piston further into the neck. If no displacer bar is inserted, the second piston admittedly necessarily receives a charging by the fluid in the end position of the first piston, but it moves only slowly up to an abutment in accordance with a restrictor point lying therebefore.
In another solution the second piston is charged from the very beginning with the fluid which is under pressure and the displacer bar, which itself is supported by the membrane at the ejection piston, prevents the second piston from moving relative to the first piston. This causes the resistance of the membrane to puncturing to be significantly greater than the greatest pressing out force during the pressing out of the bone cement from the cylindrical part of the cement syringe and the maximum force which can be produced in the encountering of the ejection piston with the transition piece to the neck of the cement syringe to be significantly greater than the resistance of the membrane.
A further possibility of producing a breaking-loose force at the displacer bar consists in a differential piston between the first and the second piston, with the differential piston taking up the pressure from the fluid over a short path with its additional piston surface and transferring this additional force directly to the second piston, which in turn moves the displacer bar.
The controlling of the fluid flow advantageously takes place with a trigger bow which is connected to a control slider.
With a fluid which enters in gas form into the control device it is difficult to use a conventional control slider since due to the low advance speeds for the ejection piston the restrictor points for the fluid flow must be chosen so small that the influence of the clearance of the control slider acts disturbingly. It therefore proved advantageous to journal a control slider with O-rings in a tube piece and to place a blocking region and individual bores in this tube piece in the longitudinal direction at such a spacing from one another that they are freed for the through-flow one after the other by an O-ring in the control slider. Since in a gas cushion which is present at the piston under pressure, the former continues to be present and to press out cement even when the through-flow of the infeed line is blocked, it is advantageous when the gas cushion which is still present is simultaneously compulsorily ventilated when the trigger bow is not actuated, i.e. when the infeed is blocked.
In order to provide the operator with a tactile feel for the pressing-out speed, he must first bring the trigger bow out of the blocking region against a weak spring into a slow pace region in which a small single bore becomes free as a through-flow cross-section. For CO2 this bore diameter can amount to 0.08 mm in order after the attaching of the cement syringe to move the bone cement slowly and in a preparatory manner up to the opening of the syringe. During the further pulling through of the trigger bow a stiffer spring comes into engagement; at the same time the number of the effective individual bores is increased. In this way it is possible to distribute the functions of blocking, slow pace and pressing out up to maximum speed over a displacement path of the trigger bow which can amount to between 10 and 20 mm and which is felt to be pleasant for the setting.